1. Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery and a method of preparing the same and more particularly, to a negative active material for a rechargeable lithium battery exhibiting good discharge capacity, efficiency, and cycle life characteristics and a method of preparing the same.
2. Description of the Related Art
A negative active material for a rechargeable lithium battery uses carbonaceous materials such as amorphous carbon or crystalline carbon. Among these carbonaceous materials, crystalline carbon such as artificial graphite and natural graphite is widely used because of its high capacity.
It is considered that natural graphite exhibits a higher charge and discharge capacity than artificial graphite, and hence satisfies the demands for batteries with higher capacity. However, high reactivity of the natural graphite limits the electrolyte. In addition, natural graphite is easily plate-shaped by pulverizing and sieving, which increases the degree of graphitization (crystallinity) to increase capacity. The plate-shaped graphite deteriorates battery performance factors.
To overcome such problems, various studies on development of a negative active material that exhibits advantages of both a crystalline carbon and an amorphous carbon have been undertaken.
Japanese Patent Application No. 2000-265417 SAMSUNG SDI CO., LTD. discloses a negative active material, including a crystalline carbon core and a semi-crystalline carbon shell formed on the core. The shell includes elements serving as graphitization catalysts.
Japanese Patent Application No. 2000-261046 discloses a negative active material which is produced by oxidize-heat treating graphite powder and shaving a surface of the resultant to break a closed structure of the surface of the graphite into an open structure followed by heat-treating it while rapidly increasing the temperature, thus re-forming a closed structure of the surface of the graphite. The graphite powder is produced by carbonizing carbonaceous materials, and pulverizing and graphitizing it. However, even though the negative active material has substantially no reactivity with the electrolyte, it has a lower discharge capacity than natural graphite.
U.S. Pat. No. 6,403,259 discloses a negative active material produced by grinding natural graphite or artificial graphite and coating the resulting material with a carbon precursor. The negative active material exhibits improved storage properties at a high temperature, better discharge characteristics at a low temperature, and it has a density of 1.20 g/cc. However, a low-crystallinity carbon surface having reactivity with an electrolyte may be present on the negative active material.